The invention relates to recording sheets suitable for use in an ink jet printing process and to coating compositions for the preparation of ink receiving layers for this process. It relates especially to recording sheets where the image recorded thereon can be observed by both reflected or transmitted light and where the ink recording sheet consists of a support onto which is coated at least one ink receiving layer and where at least one of these layers contains a mixture of water insoluble, inorganic oxides or oxides/hydroxides.
Available recording sheets for ink jet printing do not meet all the necessary requirements asked for today. In particular, there is a need to further improve their ink absorbency, ink absorption rate, image quality as well as their light stability and water fastness. A preferred embodiment of the invention relates to improved recording sheets with excellent image quality, high ink absorbency and high ink absorption rate. In particular recording sheets for ink jet printing are sought where images produced thereon show high resistance to rubbing at the surface and where the image is not altered or destroyed even under the influence of water or light.
Ink jet printing processes are of two types: continuous stream and drop-on-demand. In continuous stream ink jet printing systems, ink is emitted in a continuous stream under pressure through a nozzle. This stream disintegrates at a certain distance from the nozzle into individual droplets. The individual droplets are charged electrically according to digital data signals and deflected in a static electric field in order to direct them to a gutter for recirculation or to a specific location on the recording sheet, if this specific location on the recording sheet has to be printed.
In the non-continuous process, or the so-called xe2x80x9cdrop-on-demandxe2x80x9d systems, a droplet is expelled from the nozzle according to digital data signals and directed to a desired location on the recording sheet. A droplet is not formed or expelled unless it is to be placed on the recording sheet.
The invention is directed towards recording sheets that may be used in both recording processes.
Recording sheets for ink jet printing must meet a number of stringent demands. The printed image has to fulfill the following properties:
High resolution
High color density
Good color reproduction
Good resistance to rubbing
Good water fastness
High light stability
The following conditions have to be met to attain these requirements:
1. The ink needs to be absorbed quickly into the recording sheet.
2. The jetted ink droplets have to spread circularly on the recording sheet and have to form well-defined edges.
3. Dye diffusion in the recording sheet has to be low so that the diameter of the color areas is not increased more than necessary.
4. An ink droplet is not allowed to interfere with a droplet deposited earlier nor should it blur it.
5. The recording sheet needs to have a surface that gives high density and brilliance of colors.
6. The recording sheet has to show excellent physical properties before and after printing.
These requirements are partly in contradiction to each other. For example, a very rapid ink absorption rate may reduce the resistance to rubbing.
Starting with these requirements for recording sheets for ink jet printing ways are sought that should result in images showing the highest feasible color density combined with the highest feasible resistance to rubbing at the surface. Best results are obtained with recording sheets where at least one special ink receiving layer is coated onto a support.
Recording sheets for ink jet printing, where the ink receiving layer contains a water insoluble, inorganic, porous oxide in the form of colloidal aluminum oxide/hydroxide with pseudo-bohemite structure have been proposed in patent applications EP 0,298,424, EP 0,407,720, EP 0,622,244 and JP 60-245,588. Pseudo-bohemite is an agglomerate of aluminium oxide/hydroxide of formula Al2O3.n H2O where n is from 1 to 1.5.
Recording sheets for ink jet printing, where the ink receiving layer contains a water insoluble, inorganic, porous aluminum oxide/hydroxide comprising one or more elements of the periodic system of the elements with atomic numbers 57 to 71, have been proposed in patent application EP 0,875,394.
Recording sheets for ink jet printing, where the ink receiving layer contains a mixture of silicium dioxide and aluminum oxide/hydroxide (sol with 23% Al2O3), have been proposed in patent application WO 00-02,736.
Pigment layers for paper coatings containing a mixture of a coarse inorganic pigment (size 2 xcexcm to 4 xcexcm) and of a fine inorganic pigment, wherein the logarithmic difference of the mean particle size of both pigments is at least 0.5, have been described in patent application EP 0,777,014. Non-porous calcium carbonate or kaolin are used as pigments.
Recording sheets containing such porous oxides or oxide/hydroxides absorb aqueous inks more rapidly than recording sheets not containing such porous substances. Furthermore, such recording sheets show a tendency towards insufficient clearness and brittleness.
An objective of the invention is to provide recording sheets for ink jet printing showing high ink absorbency, high ink absorption rate and excellent image quality.
A further objective of the invention is to provide such recording sheets for ink jet printing giving excellent image quality with most ink jet printers that are available on the market today.
A further objective of the invention is to provide recording sheets for ink jet printing comprising at least one ink receiving layer containing a mixture of water insoluble, inorganic oxides of aluminium or silicium or oxide/hydroxides of aluminium, wherein at least one of these oxides or oxide/hydroxides is porous.
A further objective of the invention is a way of preparation of such ink receiving layers by using the water insoluble, inorganic oxides or oxide/hydroxides in the form of solid compounds.
This task is solved by a recording sheet for ink jet printing containing in at least one ink receiving layer in addition to one or more binders a mixture of different water insoluble, inorganic oxides of the elements aluminium or silicium or oxide/hydroxides of the element aluminium or of aluminium silicates with different chemical and/or morphological properties wherein at least one of these oxides, oxide/hydroxides or silicates is porous. The amount of this porous, water insoluble, inorganic oxide or oxide/hydroxide is at least 8 weight percent, preferentially at least 40 weight percent of the total amount of all the water insoluble, inorganic oxides, oxide/hydroxides or silicates mentioned before. The different water insoluble, inorganic oxides, oxide/hydroxides or silicates are of similar size.
xcex3- or xcex4-aluminium oxide, aluminium oxide/hydroxide, aluminium oxide/hydroxide doped with elements of the rare earth metal group of the periodic system of the elements or silicium dioxide are the preferred water insoluble, inorganic oxides or oxide/hydroxides. These substances exist in many cases in a porous and a non-porous form.
In addition, the ink receiving layers contain one or more binders.
Preferred binders are gelatin, polyvinyl alcohol, derivatives of polyvinyl alcohol, polyvinyl pyrrolidone or mixtures thereof in an amount of preferentially 5 to 50 weight percent relative to the total amount of the water insoluble, inorganic oxides, oxide/hydroxides and silicates.
Especially preferred are film forming polymers or binders in latex form.
The layers may be further modified by the addition of a cross-linking agent for the binder or binders, of fillers, of natural or synthetic polymers or other ingredients well known to someone skilled in the art that improve the pictorial and/or physical properties of the image as for example UV absorbers, optical brighteners, light stabilizers, antioxidants, humefactants, spacing agents, etc.
The invention is explained in detail in the following detailed description.